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Calc-alkaline Volcanic Rocks Petrography Processes Field relations Volcanic arcs Petrogenesis Petrography Fabric Classification Alteration Fabric Aphanitic • Rapid cooling • Loss of volatiles Glassy (vitric) • Chilling of viscous magma Clastic • Fragmented due to rapid, violent eruption Compositional Classification Rhyolite Rhyodacite Dacite Andesite Latite Basalt Textural Classification (glassy silicic types) Obsidian Perlite Pumice Vitrophyre Volcaniclastic Classification Epiclastic • Transport by earth’s hydrologic system • Volcanic sandstone, shale, etc • Greywacke • Lahar Pyroclastic Classification Components • Vitric, crystal, lithic Size • Ash, lapilli, blocks & bombs Types • Tuff • Welded tuff • Breccia Alteration Deuteric alteration • Occurs as materials cool after emplacement Hydrous minerals may decompose • Due to reduction in pressure • Fe-Ti dusty rims on reddish pseudomorphs Alteration Hydrothermal alteration • Forms due to circulation of hot ground water Propylitic alteration • Formation of hydrous minerals • Chlorite, amphibole, epidote, phrenite • Associated with some ore bodies Extrusive Processes Lava flows and domes • Thicker and shorter than for basalts • Due to higher viscosity Mono Dacite Domes lava flows Pyroclastic Processes Eruptive column Pyroclastic fall Pyroclastic flow Pyroclastic surge Ash-flow Sheets Flow units Cooling units Welded tuffs Morphology Controlled by topography Fill depressions Even upper surface Valley ponded deposits Veneer deposits Multiple lobes and fans Lateral levees Flow Unit Standard Section Layer 1 (ground layer or surge) Layer 2 (flow unit) Layer 2a (fine-grained basal) Layer 2b (main body of flow) Layer 3 (ash cloud) Welded Tuff Degree of welding • Non-welded • Partially welded • Densely welded Density is a good index Welding (density) profiles Partly Welded Bishop Tuff Partly Welded Bishop Tuff Welding Profiles Density plotted vs. elevation r = 1.0 at base and top r = maximum value near center Erosion easily removes upper part Welding = f(temperature, Pressure) Vitrophyre, Armenia Secondary Mineralization Vitrophyre Devitrification Lithophysae Vapor-phase Zeolitization crystallization Compositional Zoning Initial eruptions • Crystal-poor rhyolites • Crystal-rich latites or dacites Related to zoned magma chambers • Highly-evolved upper parts • More primitive lower parts Evidence in banded pumice Calderas Cauldron subsidence Resurgent calderas Caldera complexes Origin of silicic calderas Basaltic calderas Active calderas Collapse Calderas Atitlan, Crater Guatemala Lake, Oregon Ksudach, Toya Russia Caldera, Japan Thera, Greece Taupo, New Zealand Caldera Characteristics Simple circular form Diameter V > 2 km of collapse = V of tephra Steep walls Collapse megabreccia Generally a lake Model of a collapse Caldera Sector Collapse Scarps Horseshoe Open shape towards debris apron Gravitational Associated Van mechanism with andesite cones Bemmelen model Mount St. Helens Mount St. Helens, WA Stages in Resurgent Calderas Tumescence Early rhyolite dome Stages in Resurgent Calderas Main events • Plinian fall • Ash flow • Collapse Resurgent domes Geothermal stage Origin of Silicic Caldera Complexes Large calderas are only in continental crust Basaltic under plating plays a role Silicic magmas rise towards the surface Mixed magmas are evidence Marginal basaltic eruptions Zoned magma chambers Sequential tapping of evolving magma De Silva Model Large Composite Volcanoes Generally polygenetic Simple cones Composite cones Compound volcanoes Volcano complexes Simple Cones Single summit vent Small crater (<200 m diameter) Radial symmetry Slopes > 40o near summit Concave profiles Height of a volcano is limited May grow to 3000 m Mass eruption rate is a control Popocatepetl, Mexico Continental Rifts Afar example • Red Sea • Gulf of Aden • African rift zone Mechanism Extension or thermal anomaly first? Convergent Zones Island arcs • Oceanic/oceanic crust • Oceanic/continental crust Continental margins • Oceanic/continental crust Continent/continent collisions • Himalayan Mts. Convergent Plate Mechanisms Dipping Benioff zone Earthquakes down to 600 km depth Subducted slab • dehydrates providing rising fluids • heats as it goes down Overlying mantle wedge • partially melted by rising fluids Processes and Products Partial melting in slab and mantle wedge Fractional crystallization of magmas Assimilation of crustal material Formation of large magma chambers • calderas • batholiths Chemically evolved products Andesites and rhyolites are common Arc Volcanic Petrogenesis Magmas more felsic and diverse on continental crust • Suggests involvement of continental sial Restricted to basalt and andesite on oceanic plates • Implies mantle derived magmas Prominent Models Partial melting of the lower continental crust Partial melting of the subducting oceanic slab Melting of Peridotite in the mantle wedge
